Double heterodyne signal receiving system



June 10, 1941- W. L. CARLSON 2,245,385

DOUBLE HETERODYNE SIGNAL RECEIVING SYSTEM Filed Feb. 29, 1940 2sheets-sheet 1 1 noun "Il noun lx June 10, 1941. w. L. CARLSON DOUBLEHETERODYNE SIGNAL RECEIVING SYSTEM 2 Sheets-Sheet 2 Filed Feb. 29, 1940Patented June 10, 1941 DOUBLE HETERODYNE SIGNAL RECEIVING SYSTEM WendellL. Carlson, Haddonfield, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application February 29, 1940, SerialNo. 321,539

(Cl. o-20) 7 Claims.

This invention relates to double heterodyne signal receiving systems,and has for its primary object to provide an improved receiving systemof the character referred to wherein fundamental and harmonic beatfrequency interference from the oscillator frequencies is minimized orprevented without extensive electrical shielding of the variouscircuits.

It is also an object of the present invention to provide, in a doubleheterodyne signal receiving system, an improved relation between theoperating frequencies of the first and second oscillators Iand theharmonics thereof `with respect to the selected pass bands of the iirstand second intermediate frequency amplifiers so that a. minimum ofvariably tuned circuits are required to be adjusted when tuning thereceiver to different stations.

It is a still further object of the present invention to provide asystem of the character referred to wherein the first oscillator may betuned variably through a frequency range effective to provide arelatively high iirst intermediate fre quency permitting an appreciabledrift in the oscillatorl frequency without introducing distortion ordetuning of a received signal. For this purpose, a relatively high firstintermediate frequency is preferable, with a wider pass bandcharacteristic than the second intermediate frequency amplier whichoperates at a relatively low frequency. The second oscillator tends tocorrect the elect of the frequency drift of the rst oscillator.

It is also a further object of the present invention to provide animpro-ved radio signal receiving system of the double heterodyne typefor receiving signals in a plurality of differing input signal bandssuch, for example, as a standard broadcast band, a high frequencybroadcast band, and a television signal also in a high frequency band,and in which the fundamental frequency and harmonics of the oscillatorsare prevented from introducing interference into the receiving circuits,with minimum electrical shielding.

A further object of the invention is the provision of a doubleheterodyne signal receiving system wherein a predetermined relationbetween the oscillator, intermediate and signal frequencies is effectedwhereby objectionable heterodyne beat frequency signals are largelyavoided, and wherein the effect of frequency drift of the oscillators issubstantially cancelled.

'I'he invention will be further understood from the followingdescription when considered in ,plifiein connection with theaccompanying drawings, and its scope -is pointed out in the appendedclaims.

In the drawings,

Figure 1 is a schematic circuit V'diagram of a double heterodyne radiosign-a1 receiving system embodying the invention,

Figure 2 is a diagram sho-wing the relation of the signal, oscillator,oscillator harmonics, and

intermediate frequencies in the frequency spectrum in which the systemoperates, and

Figure 3 is a schematic block diagram of a modication of the circuit ofFig. 1 adapted for signalling reception in a plurality of signal bandsincluding television signals.

Referring to Fig. 1, input signals from a suitable source such 'as acollector or antenna 4 are applied through a tuned lter 6 to the inputcircuit 'l of a first converter provided by a balanced detectorcomprising tubes 8 and 9. The input system may include also a lowimpedance transmission line 5 coupling two parts of the filter 6 remotefrom each other. The converter may include a pair of triodes las shown,and the circuit arrangement is such that the noise-tosignal ratio andcross-modulation are reduced, while the oscillator voltage Vapplied tothe input circuit from an oscillator I0 is prevented from passingthrough the output circuit Il of the first detector to the followingportions of the receiving system to cause objectionable heterodyne beatsignals at the second converter or detector, hereinafter referred to.

The oscillator may be of any suitable type variably tunable as by avariable capacitor I2 or 'a variable inductance i3 having a movable ironor other suitable core indicated at i4. Likewise, variable push-buttontuning may be provided for the oscillator in any suitable and well-knownmanner, if desired. In the present example, the oscillator is of theHartley type comprising an oscillator tube i5, the output circuit ofwhich is indicated Iat I6, connected with the input circuit 'l of thefirst converter 8,-9, as shown.V

The output circuit Il of the rst converter is coupled to a rstintermediate frequency ampliiier comprising a tube I8 through a tunedinput transformer i9, and this is followed by a second converter 28coupled to a tuned output transformer 2i of the rst intermediatefrequency am- The second converter includes an oscillator 22 having afixed tuned oscillator circuit 23 and oscillator tube elements 24included inthe present example, in the same envelope with the secondconverter 20.

Y The output of the second converter is coupled In one embodiment of theinvention employingk a circuit in accordance with Fig. 1 for broadcast yreception, the input system 4, 5,6 is tuned to a pass band ofsubstantially E30-161() kes. and the oscillator lll is arranged forvariable tuning through a frequency range -of 5530-6510 kcs., therebyproviding a first intermediate frequency of 5000 kcs., to which thetransformers i9 and 2| are tuned. The second oscillator 22 is fixedtuned at 5260 kcs., thereby producing a second intermediate frequency of260 kcs. to which the transformers 25 and 2 are tuned.

, The frequency stability of a high frequency oscillator is ordinarilyless than that of an oscillator operating at a lower frequency. However,in accordance with the present invention, the first I. F. circuits passa Wider than normal frequency band and the second oscillator will tendto drift in the same direction as the first oscillator and in the samedirection as the resulting first I. F. frequency thereby tending to holdthe second I. F. frequency constant. Thus, if the first oscillatorfrequency-tends to drift to a higher frequency, this raises the first I.F. frequency a corresponding amount but Will Anot cause detuning for.the reason that the first I. F. circuits are provided ywith a passbandofthe order of 5 to l0 times that of the second I. F. amplifier. Thenormal tendency of the second oscillatoris to drift in the samedirection as the first oscillator which is in the direction to hold the`second. intermediate frequency constantfor impressing on the narrowband second I. amplifier. .Inthe present example, the first IF.amplifier may have a pass band `of substantially plus and minus 15 kes.,

while .thesecond I. F. amplifier may have a pass band of plus or minus 3kcs.

Referring now to Fig. 2 along with Fig. 1 the relative positions oftheVarious frequencies employed are shown in a frequency spectrum in whichthe systemoperates, the scale being drawn to cover the spectrum between100 kcs. and 90 mcs. It will be noted that the first, second and third.harmonic frequencies 35,35 and 3l, of the variable flrstoscillatorfrequency 38 do not confiict to produce objectionable I. F'. or A. F.beat interference signals at either intermediate frequency, indicated ati9 and'lill, such as 260 kes. and 5G00 kcs. or at audio frequencies.'Furthermore, the higher order harmonics than indicated are relativelyweakand do not causeV interference even though their beat frequenciesmay fall Within one of the pass bands.

The second I. F. frequency is chosen preferably to lie below the signalreceiving band, as shown. The first and second oscillator frequenciesare above the first-I. frequency. Thus, with the system arranged inthis' manner, a minimum degree of shielding of the electrical circuitsis necessary.

Furthermore, it will be noted that the harmonies of both oscillators areabove the signal receiving bandsof both the R. F. and first andv Amongthe advantages derived from the system as shown is that no variableinput radio frequency selectivity is employed. The circuit largelyavoids interference due to image signal frequencies, sum or differencesignal frequencies, and signal frequencies at the intermediatefrequencies. Furn thermore, the first oscillator is required to coveronly a 1 to 1.2 frequency range to receive the 540 to 160() kcs.broadcast band.

The circuit shown in Fig. 1 may be employed for relatively highfrequency signal reception as shown-for example, in Fig. 3, to whichattention is now directed.

Referring to Fig. 3, the double heterodyne receiving `system of Fig. 1is shown in its application for the .reception of signals in a pluralityof differing frequency bands, for example, the reception of sound'signals on the 5530-1600 kcs.

band and the 41-44 mc. band, and for the reception of a televisionchannel, both sound and picture signal, at 44-50 mc.

The input circuit comprises three band pass filters 5f), 5l, and'52 forthe R. F. signals in the three receiving bands `each of which maybe ofthe type shown in Fig. 1 and precede the first converter 5-3, to whichthey Yare selectively connected by suitable selector switching means 54,the input circuit from the signal source l being similarly controlled.by selector switch means 55, so that signals from thesource may beselected in a predetermined band and applied to the first converter 53.

Three first oscillators indicated at. 55, 5l and 58' are likewiseprovided for the 4first converter and connected therewiththroughselector switch means 59 conjointly operable with switch means5ft and 55 as indicated'by the dotted connections fill. f

It should be understood that the three input band pass filters may, ifdesired, includeV amplification and variable tuning and, likewise, thefirst oscillators, preferably .55 and 5i, and variably tunable, asindicated. The third oscillator may be kadjusted to a predeterminedfixed frequency such as 58 mcs., for the reception off-the singletelevision channel.

In the present example, the first intermediate frequency amplifierindicated at 5I is tuned to a slightly vhigher frequency than in thepreceding example .and may have a mean pass band frequency of 8.25 mcs.,as indicated, 4the pass band being plus or minus v25 kcs. for thepurpose of permitting oscillator drift, as hereinbefore de-k scribed.

For this intermediate frequency, the variably tunable first oscillator56 for the broadcast band may tune through a range of 8.78 to 9.85 mcs.,while, for the `@L44 mc. receiving range, the oscillator 51 is tunable,as indicated, between 19.25 an`dr52.25 mcs. Y

The second converter is indicatedat E52 and may include amplification,and is coupledtoa second oscillator 63 providing afixed' outputfrequency/'of 8.51 mcs. to derive from the first I. F. signal the' same260 kcs. second I. F. signal for the second I. F. amplifierfd as in thecircuit of Figpl. The band pass characteristic of thesecond I. F.amplier or filter '54 is kpreferably `adjustable' 'with change in waveband as indicated by the selector switch means 65 which iscoupled'with"selector switch means '59, 55 and 54, as indicated bythedotted ccnnectionf, for adjusting the selectivityv .or pass bandcharacteristic Yas indicatedin a range of6 kcs. to 50 kcs. for the threereceiving bands shown. It will thusbe seen that for receiv.Y

ing signals in the higher frequency ranges, the selectivity or band passcharacteristic ofthe second I. F. amplifier is changed to more nearlyequal the width of the pass band of the first I. F. amplifier for thereason that the wider band is desirable for high fidelity reception inthe ultra high frequency broadcast band.

From the first converter 53, the second amplifier channel is providedfor the video signal and comprises a band pass lter or amplifier 'IDwhich may be an intermediate frequency amplifier of which the amplifierportion is indicated at 'Il followed by a secondhand pass filter l2 inthe output circuit indicated at i3 for the video signal, cor respondingto the output circuit 14 for the audio frequency detector and usualamplifying circuits of the usual sound channel. The video channel iscontrolled by selective switching means 15 conjointly controllable withthe switching means previously referred to, to provide a connection withthe first converter when the band change switching means is moved toreceive signals in the television channel.

I claim as my invention:

1. In a double heterodyne signal receiving system, the combination ofband pass signal receiving circuits responsive to signals in apredetermined frequency band, a first converter coupled thereto, a firstoscillator for said converter tunable in a predetermined frequencyrange, a second oscillator tuned to a frequency outside of andrelatively close to said first oscillator frequency range, and signalamplifying and converting means including a second converter and a firstintermediate frequency amplifier coupled between said converters havinga predetermined pass band in a range below the frequencies of saidoscillators, said means providing a second intermediate frequency signalof substantially constant frequency in response to drift in thefrequencies of said oscillators.

2. In a double heterodyne signal receiving system, the combination of afirst converter of the balanced type, a first oscillator coupled theretoand tunable in a predetermined frequenlcy range above said band, asecond oscillator tuned above said band to a frequency outside of andrelatively close to said first oscillator frequency range, signalamplifying and converting means including a first intermediate frequencyamplifier coupled between said converters having a predetermined passband in a range below the frequencies of said oscillators and above thesignal receiving band, providing a second intermediate frequency signalof substantially lconstant frequency in response to drift in thefrequencies of said oscillators, and a second intermediate frequencyamplifier for said last-named signal tuned for response thereto in arelatively narrow pass band.

3. In a double heterodyne signal receiving system, the combination of afull wave first converter, a variably tunable first oscillator, a firstintermediate frequency amplifier, a second con verter and a secondintermediate frequency amplifier following said first .converter in theorder named, a second oscillator coupled to said second converter, meansfor tuning said first and second oscillators to provide higheroscillation frequencies than the first intermediate frequency andharmonics above the first and second intermediate frequencies, therebyto minimize beat frequency interference, and said tuning means providingoscillator frequencies so spaced in the frequency spectrum that theharmonics of the second oscillator fall between the harmonics of thefirst oscillator, thereby further minimizing beat frequency interferenceinsaid system.

4. Ina .double heterodyne signal receiving system,.the combination of afirst os'cillatorzvariably tunable through a frequency range pro--viding a relatively high first intermediate frequency signal, a firstintermediate frequency amplifier for said signal having a relativelyWide band pass characteristic, a second oscillator, a converter coupledto said second oscillator and to' said first intermediate frequencyamplifier providing a relatively low second intermediate frequencysignal, said second oscillator being tuned to a fixed frequency adjacentto the operating frequency range of the first oscillator whereby, as theoscillator frequencies drift in operation, the second intermediatefrequency is held substantially constant, a second intermediatefrequency amplifier .connected with said converter and having arelatively narrow pass band characteristic, and means for determiningthe oscillator frequencies such that the lower harmonies thereof falloutside the pass bands of said first and second intermediate frequencyamplifiers and. the lower harmonics of the second oscillator fallbetween the lower harmonics of the first oscillator, thereby to provideminimum beat frequency interference with minimum shielding.

5. A multiple channel double heterodyne signal receiving systemcomprising in combination, a first converter, a first band pass signalchannel, a second band pass signal channel, a plurality of band passlters preceding said first converter, means for selectively applyingsignals to said converter througheach of said band pass channels, meansproviding a plurality of differing oscillator signals for said firstconverter, means for selectively applying said signals to v said firstconverter to establish a predetermined intermediate frequency for saidfirst signal channel and for said second signal channel se-l. lectively,a second oscillator for said first signal .channel and a secondconverter coupled thereto and operative at a predetermined frequency toprovide a relatively low second intermediate frequency, and meanscoupled to said second converter providing an adjustable band passfilter for said second intermediate frequency, the operating frequenciesof said oscillator means being higher than the first intermediatefrequency and such that the lower harmonics thereof fall outside thepass bands of said rst and second signal channels, and the harmo-nics ofthe second oscillator fall between the lower harmonics of thefirst-named oscillators.

6. In a multiple channel double heterodyne signal receiving system, thecombination of a plurality of selectable band pass input filters, and aplurality o-f selectable tuned oscillators, said oscillators havingoperating frequencies above the pass bands of the respective band passfilters, a first converter selectively connected with said oscillatorsand band pass filters to provide relatively high intermediate frequencysignals, band pass lter means for said signals, and means for convertingsaid intermediate first signal to a relatively low intermediatefrequency below the lbands of said band pass input filters.

'7. In a double heterodyne signal receiving system, the combination ofmeans providing a band pass signal input circuit, a band pass firstintermediate frequency amplifier, a band pass second intermediatefrequency amplier, a variably for controlling the frequency of bothoscillators rtunable rst oscillator, Athe rst oscillator ybeing suchthat the harmonics thereof are outside of tunable through a frequencyrange providing a the frequencyranges established fer the signalrelatively high rst intermediate frequency and and intermediatefrequencies, Whereb-yinterfer- 'a second oscillator tuned to provide arelatively A5 ence beat frequencies are minimized. v

low second intermediate frequency, and means WENDELL L. CARLSON.

